Narrator (April Hobart, CXC): A team of astronomers has used NASA's Chandra X-ray Observatory and ESA's XMM-Newton to study a large group of galaxy clusters with a surprising result. Galaxy clusters are the largest objects in the Universe held together by gravity, and thus can reveal lots of information about the cosmos. This most recent study, which included the well-known Perseus cluster and 72 others, has uncovered a mysterious X-ray signal. Astronomers are intrigued by a spike of intensity at a specific wavelength of X-ray light in the data because of one proposed explanation. Scientists think that a hypothetical particle called a sterile neutrino may, in fact, be responsible for this spike of intensity. Some scientists have proposed that the sterile neutrino could be a candidate for dark matter, something that makes up about 85% of the Universe yet does not emit or absorb light. While they are excited about this finding, the researchers say it’s too early to claim whether or not this mysterious X-ray signal is real, or whether or not it is indeed the signature of the sterile neutrino. They'll keep gathering data and looking at other galaxy clusters to make sure they see it elsewhere. In the meantime, they’ll be looking at their theories to see where else the physics may take them.